1. Field of the Invention
The present invention relates to a wafer handling system and method for use within a lithography system. More particularly, this invention relates to a system and method of wafer handling in which wafers are transported within a lithography system while being affixed and aligned to chucks, thereby maximizing production throughput.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized. Through increases in both alignment precision and projection accuracy, today's lithography tools are capable of producing devices with ever decreasing minimum feature size. However, minimum feature size is but one measure of a lithography tool's utility. Another critical measure is throughput.
Throughput refers to the number of wafers per hour that can be patterned by a lithography system. Every task that must be performed on wafers within a lithography system contributes to the total time required to pattern the wafers, with an associated decrease in throughput. One critical task that must be performed repeatedly within a lithography system is wafer alignment. Wafers must be precisely aligned within a lithography system in order to achieve high levels of overlay accuracy. Unfortunately, alignment precision is usually lost whenever wafers are moved within conventional lithography systems with robots.
What is needed is a system and method for handling wafers within a lithography system that both avoids the loss of alignment caused by conventional robots, while at the same time improving system throughput.